Faired cables have been well known and used for underwater towing operations of all sorts. In particular, faired cables are used for such underwater towing purposes as submarine detection, and other military uses and purposes such as a deployed radio antenna from a submarine; and for other purposes such as petroleum, trace element or geophysical underwater exploration. In all of these cases, a towed body (or buoy) is towed behind a moving vessel, with the cable between the vessel and the towed body generally assuming or adopting a catenary curve. It is also well known that faired cables generally provide a much better performance by reducing the resistance to motion, or cable drag, of the towed cable as it moves through the water.
However, it has been found in the past that most fairings have certain tow-off or kiting characteristics. These come about when the fairing is not perfectly aligned with the relative flow, so that the fairing tends to act like a wing creating side lift, by which the fairing and the cable with which it is associated moves to the side, thus creating kiting or tow-off.
It has been the general intention that fairings that are assembled onto a cable will be freely mounted so that they may articulate about the cable, with the result that ony a small hydrodynamic restoring force is needed to drive the fairing into alignment with the flow. Unfortunately, however, it has been the general experience with highly streamlined fairings--which are generally light weight plastic, having flexible noses--that there is a frictional resistance or stiction at the interface between the cable and the fairing, as well as interference between adjacent fairing segments. The interference between adjacent fairing segments may particularly be caused by an axial component of the hydrodynamic drag forces, which causes the fairing segments to be stacked in compression, and which cause the frictional resistance to occur and thereby precludes free articulation of the fairing on the cable. In some cases, the fairing may stall or lock onto the cable at some angle to the direction of flow; and the mis-alignment of the fairing may adopt a wedged or a saw tooth configuration which is hydrodynamically disadvantageous.
The prior art comprises several patents of special interest, including the patent which teaches the most generally accepted plastic fairing having a flexible nose, and which is widely used in both military and commercial operations. That patent is HALE et al U.S. Pat. No. 3,611,976, issued Oct. 12, 1971, assigned to the same assignee as the present invention. The previous HALE et al patent teaches a structure by which nose and tail portions of the fairing are assembled in line with one another, and in which there is a flexible link provided between each adjacent pair of fairings. Moreover, the Hale et al patent teaches a successful fairing structure which has a flexible nose capable of being flexed in the forward direction so as to be reeled and unreeled over a sheave and onto a winch or drum. In some respects, fairings according to the present invention have a similar general appearance to the fairings taught by the Hale et. al. U.S. Pat. No. 3,611,976.
ARMSTRONG, in U.S. Pat. No. 3,092,067, issued June 4, 1963, teaches a tandem fairing which, when mounted to a cable, includes what is essentially only a tail portion that is secured around a cable by a plurality of clips along its length. However, of interest in the Armstrong patent is the fact that a number of rings are provided along the length of the cable which are intended to preclude substantial longitudinal motion of the fairing relative to the cable. However, the Armstrong fairing does not sufficiently streamline a cable so as to reduce the cable drag, neither is it practical in the sense of reeling and unreeling.
ANASTASIO et al, in U.S. Pat. No. 3,407,777, issued Oct. 29, 1968, teach a ring which is bonded around the circumference of a cable for the purpose of supporting cable fairings and the like. Anastasio et al have noted the tendency of certain fairings to kite, and provide a complicated fairing structure having stainless steel nose pieces and a polypropylene tail section, with alignment rods between adjacent sections, having a ring mounted at the centre of each fairing section. This not only provides a very heavy and cumbersome structure, it is very costly to assemble and extremely difficult to repair in the field. Moreover, because of the nature of the fairing structure, having bevelled ends on the steel nose sections, there is turbulence and therefore not as efficient reduction in drag as is possible using the Hale et al fairings referred to above.
CALKINS, in U.S. Pat. No. 3,352,274, issued Nov. 14, 1967, provides a theoretical explanation of the effect of hydrodynamic forces in restoring a fairing to be in alignment with respect to the direction of flow. However, Calkins discusses a theoretical hydrodynamic centre, in a substantially solid structure, having only a pair of cavities throughout the length of the fairing to accommodate what is, in essence, a twisted pair of wires that do not contribute to the structural design of the overall towed structure. In other words, the fairing structure is also a tension structure. For most deep towing, and military purposes, Calkins does not provide a practical structure, and fails to appreciate the problems that are presented when towing an underwater body at the end of a cable which may be up to several thousand meters in length.
RATHER et al, in U.S. Pat. No. 3,233,571, issued Feb. 8, 1966, provide a large fairing structure which may be instrumented, and which has a shoulder for each fairing secured to a cable so as to preclude longitudinal displacement of fairing structures along the cable towards the towed body. The structure of the Rather et al, fairing is also provided with a cantilevered weight forward of the leading edge of the fairing, so that it may not be reeled without first removing the fairings from the cable.
SILVEY, in U.S. Pat. No. 4,075,967, issued Feb. 28, 1978, provides a fairing section having a nose portion and a trailing body portion, with a plurality of veins which are upstanding from the trailing body portion. Silvey also provides spacer rings along the cable, one for each fairing section, which act to prevent the fairing sections from sliding down the cable and which thereby act to prevent the fairing sections from crushing one another at the bottom of the cable when they are being towed.
Some of the prior art patents, include the use of shoulders, spacer rings or supporting rings, and in patents the rings are employed so as to preclude relative motion of the fairing segments above them downwards along the cable. Such rings are otherwise known as "stacking rings", and in all of the prior art the rings act in such a manner that the fairing elements above them are compression loaded against them due to the axial component of the hydrodynamic forces acting upon the fairings.
However, as noted, there are primarily two causes of two-off or kiting, and they are the frictional interference between the cable and the fairing (stiction), and the axial intereference between adjacent fairings on the cable. Both of those causes will occur in any of the prior art fairing assemblies, and particularly the second cause will occur in any assembly where stacking rings are used, against which a plurality of fairings are "stacked" or forced against one another in compression. In any prior art assembly using rings which are secured to the cable, except in those instances where one ring is used per fairing element, the plurality of fairing elements above the ring are forced into compression against the ring.
Other difficulties that have arisen with prior art fairing assemblies for underwater towed cables include the fact that it is necessary to provide for reeling and unreeling of the towed cable over sheaves and onto winches or drums. This is provided for in the case of the Hale et al prior art fairing by the flexible nose; and in some of the other prior art fairings by the provision of a shaped steel nose, having a general curved configuration in the leading edge of the nose which approximates the curvature of the sheave or drum.
However, occasionally excessive radial torque will occur, as well as excessive lengthwise torque along the longitudinal axis of the cable and the fairings, such that a tail portion of a fairing may break away from its nose piece because of the shear forces generated. A principal object of this invention is to provide a means in the fairing structure that precludes such failure, and therefore assures the structural integrity of the fairing assembly, without the occasional loss of a tail portion of a fairing.
Moreover, yet another improvement is made by the present invention that causes the natural bend radius for the fairing assembly during reeling and unreeling to be the same as that of the cable. This further change also provides an additional compliance, so that the tail portions of a fairing assembly may spread apart one from another during reeling or unreeling, without undue stress occuring in the nose portion of the fairing assembly.
There may also arise from time to time and particularly depending upon the physical installation and the manner of the run which the faired cable must make from its winch or drum to a sheave and thence underwater, the need for the fairing orientation to be turned over by 180 degrees within a distance of as little as a few meters. When fairing tail portions are related one to another by flexible links passing between them, or by virtue of being assembled or integrally constructed with their nose portions, it is clear that some means must be provided to permit torsional compliance of a group of tail portions so as to permit re-orientation of the tail portions over a short distance. Over a long distance--say 50 meters--a gradual torsional displacement of the tail portions through 180 degrees may occur, but this may not be possible due to the physical constraints of space alloted for the winching and reeling apparatus for the towed cable on board the vessel.
This invention recognizes that there are certain conditions when it may well be desired to place groups of fairings on a cable in a compression loading relationship, and in so doing the present invention provides a structure by which such groups of fairings may be defined and installed on a cable relative to a stacking ring that is assembled to the periphery of the cable. The fairings in such asssembly overcome the difficulties previously spoken of, particularly the tendency to lose tail portions during reeling or unreeling, and as well the present invention provides a means whereby limited relative motion of the fairing segements along the cable between stacking rings is permitted.
In a more general application, however, and contrary to the prior art approaches, the present invention provides a means of suspending groups of fairings along a towed cable by off-setting the tail portions with respect to the nose portions in any group; so that relative motion of the fairing groups downwards along the cable due to the usual stacking forces causes the fairings within each group to be placed in tension and to spread apart one from another. There is therefore no tendency of adjacent fairings, or indeed of adjacent fairing groups, to bind with one another at the interface between them. In each fairing group there is a tensile string of fairings that is thereby created, with the ring which is secured to the cable in each group acting as a suspension ring so as to support the downward load of the fairings below the ring, rather than above the ring as before.
Moreover, the present invention also provides for a unique assembly of a stacking or suspension ring onto the periphery of a cable--particularly an unsheathed cable--whereby motion of the ring per se along the cable is substantially precluded. Moreover, the ring of the present invention, and the tool by which it is applied, are such that application or repair as necessary may be made in the field (such as aboard ship).
The improved fairing of the present invention substantially precludes the tendency to occasionally lose a tail portion during reeling or unreeling, and has a nose portion which is flexible and which has a substantially circular opening throughout its length, where the nose portion is split along its length to the rearward side thereof so as to permit it to be placed over a cable, and where the rear portion has a pair of rearwardly extending legs and a pair of flanges which extend sideways, one from each leg, with one leg and one flange at each side of the split. The tail portion comprises a pair of separable and substantially identical half pieces, each of which has an inner and outer surface, and each of the half pieces has at least one locating pin at the forward edge of its inner surface, which locates in one of a plurality of mating recesses formed in the legs of the nose portion, forward of the flanges. The outer surfaces of the half pieces, when assembled to the nose portion, have a substantially smooth surface in the general shape of an airfoil. Each of the half pieces of the tail portion has at least one supporting pin and a meeting recess formed on its inner surface, so that a pair of identical half pieces will mate with their supporting pins and recesses in co-operating relationship when the pair of half pieces are placed with their inner surfaces facing each other. A channel is formed along the length of each half piece rearward of the at least one locating pin, so as to receive at least a lengthwise portion of one of the flanges of the nose portion, and there is at least one shear block formed in each channel. Each shear block has a shear strength in the lengthwise direction of the fairing, higher than the shear strength of the locating pin; and there is at least one discontinuity in each flange of the nose portion to accommodate the respective shear block of the mating tail portion half piece.
A particular advantage of the above-described fairing configuration, that is has considerably less tendency to lose a tail piece or a half tail piece during reeling or unreeling of a faired cable assembly, comes particularly because of the incorporation into the fairing structure of shear blocks, and is discussed in greater detail hereafter.
Moreover, an assembly of fairings such as those described above can be easily made, whether the fairings are placed either in compression or preferably in tension, and includes a ring which is secured to the periphery of the cable at at least one point along the length of the cable, where the ring has a length less than the length of any tail portion of any fairing. A first tail portion is adjacent the ring, with the forward end of the first tail portion being mated to first and second nose portions, each having a length which is less than the length of any tail portion. The first nose portion has a rearwardly directed edge which is remote from the ring and which is flush with the edge of the first tail portion, and the second nose portion overlies a portion of the length of the first tail portion and a portion of the length of the second tail portion adjacent to the first tail portion. A plurality of nose portions and mated tail portions is then mounted along the length of the cable, with the cable being accommodated in the lengthwise opening of each of the nose pieces. At least periodically along the length of the faired cable assembly, there are adjacent tail portions which are joined one to the other by a flexible link piece which has an opening formed at each end thereof, each of the openings being of sufficient size to accommodate a supporting pin which is in one of the adjacent tail portions; and there is an opening through at least the facing ends of the adjacent tail portions so as to accommodate the flexible link which is passed therethrough.
When the assembly is placed in compression, as discussed above, the tail portions other than at the first and second nose portions and the ring, are assembled directly behind their respective nose portions, and a flexible link is secured over the supporting pins between each adjacent pair of tail portions. In such assembly, the plurality of fairings and the first nose portion are above the ring which is secured to the cable, and the second nose portion is below the ring, when the cable is deployed underwater for towing.
On the other hand, when it is desired that the fairing assembly be placed into tension, as is the usual case in this invention and which is in contradistinction to all prior art assemblies, the fairings are assembled other than at the first and second nose portions and the ring, into groups of fairings where the tail portions within each group are in stepped or staggered fashion with respect to the nose portions within that group, and having a nose portion of one half length of the other nose portions assembled at the top and bottom ends of each group, with a flexible link secured over the supporting pins between the topmost and bottommost tail portions of each adjacent pair of groups of fairings. In the tension assembly, the groups of fairings and the first nose portion are below the ring, and the second nose portion is above the ring, when the cable is deployed underwater for towing.
The present invention provides a novel ring assembly for securement to the periphery of cables, particularly unsheathed cables, which comprises a circular, hollow cylindrical collar of a compressible synthetic rubber-like material, having a slit from one end to the other so that it may be placed over a cable. The inner circumference of the collar is substantially equal to the circumference of the cable, and there is an adhesive layer in the inside surface of the collar. A hard brass outer sleeve which is also of generally circular and hollow cylindrical configuration is provided, and the outer sleeve also has a slit from one end to the other end thereof so that it may be put in place. There is a chamfered inner surface at one side of the slit, and a chamfered outer surface at the other side of the slit, with the chamfers each being substantially identical to each other. The circumferential length of the outer sleeve from one side of the slit to the other side is greater than the other circumference of the collar, so that when the brass sleeve is placed over the collar, the chamfered surfaces face and overlap each other. A solder coating is provided on each of the chamfered surfaces; and when the soldered coating is heated using a tool which is also provided by his invention, a solid sleeve is obtained. The tool comprises a hollow barrel to accommodate hot air blown thereinto, and a heating foot at the end of the barrel which is adapted to be heated by the hot air and has vents formed therein for the hot air to exit away from the heating foot and barrel, and clamp means to clamp the brass sleeve against the heating foot which is curved so as to accommodate the same when the tool is used, hoop stress is created within the brass sleeve and compression forces are created by the brass sleeve against the collar. After the brass sleeve is heated and the chamfered surfaces are soldered to each other, and after the assembly has cooled down and the tool is disassembled, the hoop stress and compression forces remain in the brass sleeve.
There also occurs certain circumstances where it is desirable that the flexible link which joins adjacent groups of fairings in tension, or adjacent fairings when in compression, has an elastic memory. This permits the adjacent tail portions joined by the link to move apart from each other at times, and to be restored to their more general proximal relationships at other times, due to the elasticity of the link. Generally such link is formed in the shape of a U, where each leg of the U is installed in the tail portion of one of the pair of adjacent tail portions that have the flexible link between them.
A further embodiment of fairing is also provided by this invention, and it is intended for use in association with other similar fairings, where the fairings may generally be used in a shorter faired cable assembly or in situations where it is desirable that the fairing may be assembled and disassembled from the cable relatively quickly and without destruction of the fairings. The further embodiment of fairing comprises a nose portion and a plurality of tail portions which are constructed integrally with one another, where a substantially circular opening is formed in the nose portion throughout its length, and is defined in its forward portion by the inner surface of the forward portion of the nose, and at its rearward portion by inwardly directed curved shoulders which are formed inwardly from the sides of the fairing; and where the shoulders are split from each other. The plurality of tail portions are each formed integrally with the nose portion and extend rearwardly therefrom, with the tail portions being separated from each other by slits which extend rearwardly from the region of the shoulders to the end of the fairing. The outer surface of the fairing is substantially smooth and in the general shape of an airfoil, except at the slits. The walls of the fairing are of substantially constant thickness except in the regions of the curved shoulders, and the fairing structure is split at its rearmost end so that the sides may be separated from one another so as to place the fairing over a cable, with the cable being accommodated in the substantially circular opening throughout the length of the nose portion thereof. The material of the fairing has elastic memory, so that when it is placed over a cable and the sides are released, it will restore itself to its configuration where the rearmost ends of both sides of the fairing are in close proximity to each other. Moreover, the nose portion of the fairing may be flexed in a forward direction such as when the faired assembly is reeled or unreeled over a sheave or onto a drum or winch, but it is capable of restoring itself to a substantially straight configuration when deployed underwater.
Normally, at least one slot is formed in the forward portion of the nose portion of the fairing described immediately above, so as to accommodate the ring secured to the periphery of the cable. The alternative fairing embodiment just described is normally formed of rigid, extruded polyurethane or vinyl.
On the other hand, the first embodiment of fairing which is provided by this invention comprises injection moulded components, where the nose portion is normally formed of a polyurethene, and the half pieces of the tail portion are formed of a more rigid material such as acrylonitrile-butadiene-styrene.
One of the practical problems that occurs in preparing faired cable assemblies is the fact that there is a relatively wide range of tolerances with respect to the diameter of the cable to be faired, as well as the likelihood of the existence of a raised or projecting strand of armour, such that the surface of the cable may become a somewhat unreliable bearing surface. Moreover, the fairings themselves are also subject to certain manufacturing tolerances, so that the nominally circular opening through the nose portion of the fairing through which the cable passes may, in fact, be slightly oval in shape from time to time.
The present invention recognizes and overcomes these problems by the provision of a fairing system that can tolerate the existence of a relatively large clearance between the fairing and the cable--in the order of 5 to 10 percent of the cable diameter. Moreover, the present invention provides a faired cable system that, especially when the fairing groups are in tensile strings rather than compression columns, there is a greater tendency of the fairings to align themselves with the direction of flow due to the apparent forward movement of the centre of rotation of the mass of the fairings, especially at small tow-off angles.
By providing a fairing assembly, especially a tensile string which is suspended in discrete lengths from the suspension rings that are secured to the periphery of the cable, a faired cable assembly is provided that allows for unimpeded articulation of the fairings around the cable. Moreover, when the fairing segments are flexibly connected to each other, and the interconnection between the nose portions of the fairing segments is resilient, and also the fairing segments themselves are resilient both in bending and in torsion, the axial hydrodynamic forces that are developed are successfully resisted. This allows the fairing tail portions to remain free from frictional interference with one another, and they may thereby utilize any restoring forces that they may receive as the faired cable assembly moves through the water to maintain them in line with the direction of flow.
There are thus provided by the present invention several embodiments of fairings which can be placed in compression or tension when in assembly with other fairings; and especially an embodiment of a fairing for use in a reasonably permanent installation in a faired cable assembly which can, however, be assembled and/or repaired in the field.
An object of the present invention is, therefore, to provide fairings which may be placed in tension as well as in the usual compression arrangements, which can be easily installed and repaired in the field, and which do not have any tendency to lose tail portions during reeling or unreeling.
Moreover, the present invention provides alternative embodiments of fairings which can be produced using simple plastics injection and extrusion moulding techniques, without the necessity for extensive machining operations, and without the necessity for the use of any metallic nose or tail fairing components.
Yet another object of the present invention is to provide a stacking or suspension ring which may be easily and quickly secured to the periphery of a cable, especially an unsheathed cable, even in the field, and which retains the hoop stress and compression forces that are created or induced in the ring components during its assembly.
A tool for the installation of the stacking or suspension ring is also provided by the present invention.